1. Field of the Invention
The present invention relates to a method for data-flow quality control in a Digital Subscriber Line communications system as described in the preamble of claim 1, the related receiver as described in the preamble of claim 4 and the related transmitter as described in the preamble of claim 5.
2. Description of the Related Art
Such a method and related devices are already known in the art, e.g. from the section “AOC on-line adaptation and reconfiguration”, pages 119-123 of ITU-T Recommendation G.992.1 with title “Asymmetric digital subscriber line (ADSL) transceivers” published in June 1999 by the International Telecommunications Union (ITU).
Therein, it is described that the transmit power in a Digital Subscriber Line system, called an ADSL-system, is distributed over all data-carriers, called sub-carriers, resulting in a certain Power Spectral Density (PSD) value for each carrier. This transmit power is attenuated by the travelling along the channel and results in a certain, reduced, receive power at the receiver. Moreover, the channel also has the characteristic to add noise to the signal. From this attenuated signal together with the added noise a Signal to Noise Ratio can be determined. Based on this Signal to Noise Ratio, the number of bits that can be transmitted on each carrier while still guaranteeing a certain maximum error-rate can be determined. This number of bits then is transmitted on each data carrier by the transmitter once the transmitter is in showtime.
As the channel characteristic due to certain conditions, such as new upcoming disturbers or weather conditions and noise changes in time, the Signal to Noise Ratio of the signal changes also. Hence, due to the changing channel characteristics and noise, the current Signal to Noise Ratio may differ from the Signal to Noise Ratio that is required to transmit the previously determined number of bits per carrier while still guaranteeing a certain data-flow quality for instance a certain maximum error-rate. In order to retune the Power Spectral Density value for each carrier to a new value that fits to the changed current channel characteristics and noise on the channel without interrupting the data-flow, to guarantee a stable communication, a bit swap is performed.
Because of the fact that in ADSL version 1 as described in the above mentioned ITU-T Recommendation G.992.1, the bit swap mechanism is only able to change at most 4 (normal bit swap request) or 6 (extended bit swap request) tones every second, the modem can go out of showtime before having the time to perform all the necessary bit swaps if the noise conditions on the line are changing rapidly.
Furthermore, in ADSL version 2 as described in ITU-T Recommendation G.992.2 the bit swap mechanism is bigger and may include all carriers in the bit swap, which constitutes an improvement compared to the ADSL version 1. In this case however, the bit swap request message to be transferred is large and if the noise on the line is high, the probability that the received bit swap request message is corrupt is still considerably present.